U.S. Pat. No. 4,979,477 discloses an ignition circuit of the kind referred to above wherein the ignition time point is computed in dependence upon external operating parameters or is read out of a memory in order to obtain an optimal ignition adapted to rpm. If the work tool becomes blocked in the full-load position, the clutch slips and the rpm drops notwithstanding the full-load position of the throttle lever. Because of the load position of the throttle lever, the ignition circuit changes the ignition time point in the direction of an increase of rpm so that the engine rotates for a longer time at an actual rpm higher than the clutch-engaging rpm although the work tool is at standstill. The danger is then present that the slipping clutch will become thermally overheated and inoperable; at the very least, an operating state of this kind is very wear-intensive to the friction coatings.